Overview
This lecture explains the knee-jerk (patellar) reflex, illustrating how it works through a simple reflex arc to help maintain posture and balance.
The Knee-Jerk Reflex Explained
- The knee-jerk reflex is also called the patellar reflex.
- It helps maintain posture and balance by causing the lower leg to kick outward when the patellar tendon is tapped.
- The reflex is commonly tested by doctors to check for nervous system health.
Reflex Arc Mechanics
- The reflex arc occurs at the spinal cord level, not involving the brain in initiating movement.
- Brain receives information about the movement after it starts.
- When the patellar tendon is stretched, stretch receptors (muscle spindles) in the quadriceps muscle detect it.
- Muscle spindles activate sensory neurons that travel to the spinal cord.
Neural Pathways
- Sensory neurons synapse with motor neurons controlling the quadriceps in the spinal cord.
- Motor neurons cause immediate contraction of quadriceps, moving the leg.
- The knee-jerk reflex is monosynaptic, involving direct connection between sensory and motor neurons.
- In practice, many sensory neurons activate multiple motor neurons.
Inhibition of Opposing Muscles
- Sensory neurons also stimulate interneurons that inhibit motor neurons of opposing muscles (e.g., hamstrings).
- Inhibition of opposing muscles allows unopposed contraction of the quadriceps.
Key Terms & Definitions
- Knee-jerk (Patellar) Reflex — an automatic response where the leg kicks forward after the knee tendon is tapped.
- Reflex Arc — the neural pathway controlling a reflex action.
- Muscle Spindle — stretch receptor in muscles detecting muscle lengthening.
- Monosynaptic Reflex — a reflex involving only one synapse between sensory and motor neurons.
- Interneuron — a neuron that connects other neurons within the central nervous system.
Action Items / Next Steps
- Review diagrams of the knee-jerk reflex arc.
- Understand key terms for future exams.